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Sinkholes may provide earthquake insights

A study shows that surface warping could be a sign of active faults deep below.

March 06, 2009|Jia-Rui Chong

A study published Thursday by California researchers provides the clearest picture yet of the subtle, slow-motion warping of the earth's surface that can happen after an earthquake on a buried fault.

The research focused on a magnitude 6.6 earthquake in Iran and details how seismic forces deform the ground. Scientists said it can help us better understand the myriad faults in Southern California.

Thomas Jordan, who directs the Southern California Earthquake Center and was not involved in the study, pointed to last year's Chino Hills earthquake, where the rupture in the earth did not appear on the surface.

"Sometimes what you see at the surface is not the same as below," Jordan said. The sinking noted in the study, he said, "may explain those kinds of discrepancies."

The earthquake that struck Bam, Iran, in 2003 killed about 30,000 people. Eric J. Fielding, a geophysicist at NASA's Jet Propulsion Laboratory who led the study published in the journal Nature, said colleagues went into the area and saw nothing more than minor cracking.

They had expected more, he said. How could such a large rupture stay buried?

The scientists started collecting satellite radar images that showed elevation changes on the surface. No satellites from NASA were yet collecting that kind of information, so they used a European Space Agency satellite.

The group collected the images for three and a half years and were able to detect a narrow finger of land about 5 miles long and 1,000 feet wide sinking 1.2 inches. The scar was right above the area where the buried fault slipped about two miles down, Fielding said.

They believe the depression formed because energy unleashed by the fault cracked rocks under the earth's surface. Over time, the weight of rocks above caused the cracked rocks to collapse, he said.

It would be a stretch to think this kind of high-resolution imagery could help us predict earthquakes, said Ken Hudnut, a U.S. Geological Survey geophysicist who was not involved in the study.

But combining information from satellites and GPS monitoring stations on the ground could help identify which buried faults are active, he said.

"In Southern California, we know there are faults that are active down there that have no surficial evidence," he said. "We have to keep chipping away at the problem."


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